1. Field of the Invention
This invention relates generally to control of polyphase motors and, more particularly, to spin-up and speed control of disk drive spindle motors.
2. Description of the Related Art
Rotating direct access storage devices (DASDs), such as magnetic disk drives, are popularly used for storage and retrieval of recorded digital information in computers. The digital information comprises data that is read from or written to a circular, relatively rigid disk. The disk is rotated about a spindle by a spindle motor. As the disk is spun, a servomechanism of the DASD positions a transducer across the surface of the disk. The transducer comprises a magnetic head that can sense the digital information recorded on the disk surface. The digital information is recorded on the disk in a series of concentric tracks, and the servomechanism moves the transducer to a desired track in a data seek operation, while making the very minute movements necessary to maintain the transducer centered over a desired track in a track following operation. A DASD may incorporate more than one disk, and the DASD may also include multiple heads, one for each disk of the DASD.
The digital information stored on the DASD can include programming code that implements an operating system of the computer associated with the DASD. It is necessary to load the programming code into memory and execute the operating system in an initialization or "boot-up" sequence when the computer is first turned on. Hence, when power is initially applied to the computer and the DASD, the spindle motor of the DASD undergoes "spin-up" to bring the disk up to operating speed and permit the reading of the operating system programming code. Often, the disk will remain spinning for as long as the computer is turned on, and some users leave their computers turned on for extended periods of time. The DASDs in most computers, however, are typically shut down after a period of non-use, even if power continues to be applied, to conserve energy and improve efficiency. The DASD spindle motor then spins up at the next disk access of data.
Thus, there are many occasions on which spindle motors must quickly come up to a steady-state operating speed, which for a server class DASD is presently on the order of 10,000 revolutions per minute (RPM). Other computer drives may have somewhat slower speeds. Such rapid accelerations may occur under a wide variety of environmental conditions, particularly for laptop and notebook computers. For example, laptop and notebook computers may be exposed to a relatively wide range of temperature extremes. Access times for retrieving or recording data from the disk are measured on the order of milliseconds. Therefore, it is critical that disk spindle motors have the ability to quickly come up to operating speed under a wide variety of conditions. In addition to the requirement for fast acceleration, the drives must immediately maintain the desired speed within tightly specified tolerance values to the read and write data.
The spindle motor of a DASD is typically constructed as a brushless three-phase inductive motor. A three-phase motor is typically constructed with a rotating magnet assembly, or spindle, and a stationary coil assembly, also called the field or stator. The coil assembly is also referred to as the armature, and includes multiple inductive coils. An alternating current (AC) supplied to the coils creates a force that rotates the spindle.
At any time, current is being supplied to two of the three phases, or coils, by applying a voltage across two of the three (externally accessible) terminals. The third terminal is left floating, and therefore the voltage seen on this third terminal is simply the voltage induced by the movement of the rotating magnets relative to the coil--a sinusoidal voltage signal called the electro-magneto force, or BEMF. Current is applied to the motor windings in the manner described above in six different configurations, or states. There exist six states because there are three terminals and two polarities of current direction for each pair of coil terminals. The motor is driven by sequencing repeatedly through these six states. A commutation is the act of switching from one state to another. An "upper" commutation is one in which the terminal previously driven with high voltage is allowed to float, and the terminal previously left floating is now driven with high voltage. A "lower" commutation is one in which the terminal previously driven with low voltage is allowed to float, and the terminal previously left floating is now driven with low voltage. The six-state sequence alternates between upper and lower commutations.
The timing of the commutation for a motor phase is expressed in electrical degrees relative to the BEMF, and is an indicator of motor operating efficiency. For maximum efficiency, the windings of a phase should be driven symmetrically with respect to the BEMF waveform for that phase. For example, for a given phase, one upper commutation will begin driving current through that phase. The next consecutive upper commutation will stop driving that phase and allow it to float. For maximum efficiency, these two commutations should occur equidistant (in electrical degrees) from the maximum value of the corresponding BEMF waveform for that phase.
Motor efficiency also is indicated by the torque constant, represented as K.sub.t and specified in Newton-meters per amp. The greater the K.sub.t, the more efficient the motor. That is, the K.sub.t value represents the torque obtained given a unit current input. The torque constant K.sub.t is generally a function of the number of stator windings and the strength of the magnets within the rotor. Thus, a greater K.sub.t value indicates more output torque per input current and therefore indicates greater efficiency. Unfortunately, for a greater K.sub.t value, there is less voltage headroom for the drive currents. The motor voltage headroom is an indicator of the current reserve that can be called upon to deliver needed current to the field windings should there be an increased demand for output, or torque. Voltage headroom is also a function of power supply voltage, motor winding resistance, and motor-driver circuit resistance.
Voltage headroom is desired for a motor because, for example, there is an increased demand for torque at motor start-up, especially at relatively cold operating temperatures. The increased torque demand occurs because, for example, grease around the spindle shaft will thicken at colder temperatures and tend to clump, resisting smooth, fluid movement. This type of grease slump around the spindle shaft will increase drag on the motor spindle and require greater torque output than ordinary to get the motor to its steady-state operating speed, as well as to maintain that speed precisely. The severity of the grease slump will vary with environmental temperature and elapsed time since motor start. As noted above, this variation generally will be more pronounced, and more debilitating, in the case of laptop and notebook computers. For all computer applications, this variation in slumping will be most critical at disk drive start-up.
From the discussion above, it should be apparent that there is a need for a disk drive motor that has a high operating efficiency to minimize power consumption and that can deliver the increased torque required at disk drive start-up and under certain operating conditions. The present invention fulfills this need.